As a method of deterioration diagnosis for an oxygen sensor which is used in feedback control for the air/fuel ratio of an engine, a method of diagnosing deterioration of the oxygen sensor based upon the period of air/fuel ratio variations detected by the oxygen sensor is disclosed for example in Tokkai Sho 61-192832 published by the Japanese Patent Office in 1986. According to this method, the period, according to which the air/fuel ratio varies upwards from rich to lean past the stoichiometric air/fuel ratio and in the reverse direction, is measured from the output of the oxygen sensor, and this measured period is compared with the period characteristic of steady operation of the engine which is determined in advance; and it is deemed that deterioration of the oxygen sensor has occurred if the former is greater than the latter.
In other words, deterioration of the oxygen sensor which detects the air/fuel ratio is determined from the responsiveness of the actual air/fuel ratio to air/fuel ratio control.
Certain control variables must be determined appropriately in order to obtain desirable control responsiveness for the air/fuel ratio feedback control of the engine. In this connection, it is disclosed in Tokkai Sho 62-147034 published by the Japanese Patent Office in 1987 to provide oxygen sensors both upstream and downstream of a catalytic converter which purifies the exhaust gas of the engine, and to perform air/fuel ratio feedback control based upon the output of the oxygen sensor on the upstream side, while on the other hand varying the control variables for this control based upon the output of the oxygen sensor on the downstream side.
In this case, the period of variation of the air/fuel ratio as detected by the upstream side sensor is affected by change of the control variables. Accordingly, with this type of air/fuel ratio feedback control apparatus, if diagnosis of deterioration of the oxygen sensor is performed from the period of variation of the air/fuel ratio, it is difficult to provide a sufficiently accurate diagnosis.